As an X-ray detector used in an X-ray CT apparatus, a photon-counting X-ray detector has been known. The photon-counting X-ray detector detects each of incident X-ray beams as a photon, and counts the number of photons, thereby measuring the intensity of an X-ray. Moreover, as an electric charge is generated in an amount according to an energy of an X-ray photon when the X-ray photon is converted into an electric charge, the photon-counting X-ray detector measures an energy of each X-ray photon. Therefore, the photon-counting X-ray detector can measure also an energy spectrum of an X-ray beam.